Effect of creep temperature on Z-phase formation in heat-resistant 9% Cr–3% Co martensitic steel

Abstract

The Z-phase (CrVN) precipitation in a 9% Cr–3% Co martensitic steel during creep at 923 K and 948 K has been investigated with aim to establish the effect of temperature on the nucleation mechanism of these particles and their coarsening behavior. Ostwald ripening of VX carbonitrides strongly affects these processes. An increase in creep temperature significantly accelerates the transformation of the nanoscale MX carbonitrides into the Z-phase particles causing the Z-phase nucleation in a shorter time. Two different mechanisms of the Z-phase precipitation have been observed. Firstly, for creep tests at 923 K and 948 K with creep rupture time longer than 2000 h, the formation of the stable Z-phase particles with a tetragonal lattice occurs through in-situ transformation of a cubic lattice of the MX carbonitrides leading to a continuous flux of Cr atoms from the ferritic matrix into these particles. The coarsening Z-phase occurs at the expense of dissolution of VX carbonitrides. Secondly, for creep test at 948 K with duration of 773 h, the strain-induced metastable Z-phase with the cubic lattice nucleates on the V-rich MX/ferrite interfaces with following transformation into the stable Z-phase with the tetragonal lattice under creep testing with longer duration. Concurrently, extensive coarsening Cr-rich VX carbonitrides occurs independently. As a result, coarsening Z-phase leads to insignificant dissolution of VX carbonitrides. The creep strength breakdown appearance is not related to the formation and/or coarsening of the Z-phase at both temperatures.